Analog Synthesis

Gary Johnson

Predictably, a High-Pass Filter (HPF) allows higher frequencies to pass through and reduces lower frequencies. HPFs help sculpt brighter, more attention-grabbing sounds that can more easily cut through a mix, such as those used for riffs in electronic dance and pop music. Band-Pass Filters (BPF) preserve a range of frequencies known as the passband and reduce those. The opposite of a BPF would be a BandStop Filter. Notch Filters (NF) reduce a precise frequency range, the control of which is dictated by a controllable algorithm called the Quality Factor (Q Factor).

Amplifiers allow for a more dynamic performance by providing control over the volume and sustain of the now filtered signal and are the final building block in any analog synthesizer. The most common amplifier found in analog instruments is the Voltage-Controlled Amplifier (VCA) which responds to a control voltage to adjust the overall amplitude of a sound.

Modulation Magic

In addition to VCAs, many analog synthesizers also include Envelope Generators (EG) to supply a Control Voltage that changes over time. Not always limited to amplitude, this CV can be used to influence the frequency or other parameters of a sound. Envelope generators typically have four stages: Attack, Decay, Sustain, and Release (sometimes simply labeled ADSR). During the Attack stage, the voltage rises from zero to its maximum level. A fast or short attack time produces a more percussive start to a sound, where a longer attack generates a swelling sound that gets progressively louder. At the decay stage, the CV falls from its maximum level to that set by the upcoming Sustain stage, which holds the Control Voltage at a constant level until a Gate signal is received by a sequencer, arpeggiator or by the release of a key. It is at this point the Release stage begins, and the Control Voltage falls back to zero at the chosen rate.

EGs can also be used to modulate the frequency or filter cutoff of a sound, often employing a Low Frequency Oscillator (LFO) to add fluctuations in pitch (vibrato), amplitude (tremolo) and/or for more dramatic filter sweeps. Their flexibility and versatility make them extremely valuable when it comes to modulation, allowing for the creation of complex, evolving and expressive sounds.

Have Fun

The sonic exploration and performance possibilities of analog synthesizers are practically endless. It is hoped that the above provides a foundation in understanding, and in doing so may help to demystify this truly powerful and compelling area of sound design.